Chemical imaging is a well-known method for obtaining information about the molecular makeup of a particular material that combines digital imaging and near-infrared (NIR) spectroscopy, or other spectroscopic techniques. By illuminating the material with light of a particular wavelength, or with a broadband illumination source, and observing the light reflected or transmitted by the material at various wavelengths, it is possible to determine the composition of the material, as is well known in the art. By utilizing digital imagery in combination with NIR spectroscopy, it is possible to obtain two or three dimensional data structures which can be converted into chemical images of the surface of the sample.
The chemical imaging method includes delivering narrow band or broadband radiation to a sample and collecting the radiation that is either reflected from or transmitted through the sample via a focal plane array, a camera or raster scanned detector to yield a spectral image by modulating either the wavelength of the illumination source or the center wavelength of interference filters placed between the sample and the image detector. A complete spectral image hypercube of a sample is thus acquired in steps, wherein each pixel of the hypercube contains the optical intensity spectrum across the sample wavelength range for a specific X-Y position.
Prior art chemical imaging methods and apparatus consisted of taking point-by-point spectra of a small region of the surface of a sample and rasterizing the spectra to obtain the chemical image. This method is time-consuming and cumbersome, in that many point spectra must be collected to make a chemical image of desirable resolution.
Alternatively, chemical images may be collected using a CCD or focal plane array to collect the image over the entire desired area. Such a system is described in U.S. Pat. No. 6,734,932 (Treado, et al.), entitled “Near Infrared Chemical Imaging Microscope”. The Treado imager is an interferometric type imager which utilizes a broadband NIR or white-light illumination source, a tunable filter, such as a liquid crystal tunable filter (LCTF) or an acousto-optic tunable filter (AOTF) for wavelength discrimination, and a CCD or FPA for image capture. This type of imager, however, suffers from several drawbacks. First, broadband source illumination and inefficient light collection can have an adverse impact on the signal-to-noise ratio of the imager. Constant, broadband illumination can also often be damaging to labile samples, for example, biological specimens, thereby limiting the application of the device. Interferometric-type imagers are also limited in their ability to operate in alternative modes. For example, it is difficult to perform Stokes vibrational circular dichroism (VCD) spectroscopy, with an interferometric-type imager because the signal-to-noise ratio (SNR), optical geometry, and acquisition speed are prohibitive. Lastly, the collection of reference images for the normalization of collected chemical images can be time-consuming and cumbersome, requiring the collection of a reference image at the tunable filter for each wavelength interval of interest.
Tuned illumination type imagers are also known in the art. These type of imagers function by illuminating the samples with light of a single wavelength or a weighted combination of multiple spectral bands. Detection using these types of imagers is simplified because the need for the interferometric element (i.e., the tunable filter) is eliminated. Beam delivery is also simplified by the use of fiber optic and hollow waveguide technology. Current, prior art tuned illumination imagers utilize grating monochrometers, LEDs or laser diodes to provide single or very narrow-band wavelength illumination. Such a device is disclosed in U.S. Pat. No. 6,690,466 (Miller, et al.), entitled “Spectral Imaging System,” in which the tuned illumination source consists of an array of LEDs, with one LED per spectral channel.